93Nbmagic angle spinning NMR study of perovskite relaxor ferroelectrics(1−x)Pb(Mg1/3Nb2/3)O3−xPb(Sc1/2Nb1/2)O3

Abstract

${}^{93}\mathrm{Nb}$ NMR has been used to investigate the local structure and cation order/disorder in solid solutions of $(1\ensuremath{-}x)\mathrm{Pb}({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}){\mathrm{O}}_{3}\ensuremath{-}x\mathrm{Pb}({\mathrm{Sc}}_{1/2}{\mathrm{Nb}}_{1/2}){\mathrm{O}}_{3},$ as a function of concentration x. These relaxor ferroelectric solid solutions have been well characterized by x-ray diffraction, transmission electron microscopy, and dielectric response measurements. NMR magic angle spinning (MAS) spectra are presented for ${}^{93}\mathrm{Nb}$ at 14.0 and 19.6 T. Seven narrow peaks and two broad components are observed. Narrow peaks are assigned to configurations of ${}^{93}\mathrm{Nb}$ nuclei with only ${\mathrm{Mg}}^{2+}$ and/or ${\mathrm{Sc}}^{3+}$ (no ${\mathrm{Nb}}^{5+})$ cations occupying the six nearest B neighbor sites $(\mathrm{nBn});$ and are designated 0--6 according to the number of ${\mathrm{Mg}}^{2+}$ cations in the $\mathrm{nBn}$ configuration. The two broad components are assigned to configurations with more than one ${\mathrm{Nb}}^{5+}$ cation in the nearest B sites. In order to fit the MAS line shapes it is necessary to include distributions of the electric-field gradient parameters and dispersions in isotropic chemical shifts; these have been obtained from the data. The relative intensities of each spectral component are analyzed and the data strongly support a modified random site model of B-site order. Monte Carlo simulations of B cation ordering are presented to explain the observed NMR intensities and to validate the model.